How noisy can it really be? Most airlines are flying modern jets with quieter engines, so except for the occasional DC-9 that Delta still uses, you shouldn't hear too much provided your house has soundproofing.
Very. Jets aren't quiet. Regardless, Van Nuys is more GA (biz jets, props, helos, etc.).
I live about 10 blocks north of the approach/departure path of a major international airport and I can still hear them, albeit they are rather quiet. In fact, the props taking off fly directly over several thousand feet about and you hear those loud and clear. I can definitely see how annoying it could be for some people who may not be heavy sleepers. With that said, no one told them to right by an airport that was there long before them.
But property values are generally lower around noise disturbances like an airport as most people won't want to live near the noise. Me personally, I'd love it but you won't see me buying a house near an airport because of my interest in aviation...you could very easily be have to sell the house for less than you bought it for.